This invention relates to an improvement in an apparatus for focus-deflecting a charged particle beam.
Focus-deflection systems for a charged particle beam have generally gained wide application in charged particle exposure systems, such as systems including an apparatus for electron-beam lithography, an electron-beam milling machine, a scanning type electron microscope, an ion implanter, an image pickup tube, and so forth.
One such focus deflection system for a charged particle beam is disclosed in Japanese Patent Publication No. 14495/1973 and includes first and second deflectors of the magnetic field type spaced along the path between the object point and the specimen surface and a magnetic field lens positioned coextensive with the second deflectors. With such an arrangement operation of the first deflector has the effect of shifting the lens in a direction transverse to the beam path to thereby reduce the deflection aberration. However, even with this arrangement, it is not possible to direct the charged particle beam so that it impinges perpendicularly on the specimen surface when the beam is deflected, and so, the deflection aberration cannot be entirely eliminated with such an arrangement.
The difficulties with the formerly-disclosed system appear to stem from the combined effect of the two deflectors which require rotational adjustment to obtain a minimum aberration. This leads to a further disadvantage in that complicated adjustment procedures are required in the prior system just to achieve what is in reality a less than completely satisfactory reduction in the deflection aberration.
It is therefore an object of the present invention to provide a focus-deflection system which avoids or eliminates these disadvantages which have heretofore been inherent in systems of this type.